Rotary electric machines are used, for example, in auxiliary motors for internal combustion engines for driving hybrid electric vehicles (HEVs) and in main motors for driving electric vehicles (EVs).
Housings for these rotary electric machines are required to prevent the stator from being slipped by torque reaction or inertial force owing to vibrations acting on the stator and to dissipate or transfer heat generated at the stator conductor to outside the machine or the coolant.
It is known that a conventional rotary electric machine housing has a structure made up of a substantially cylindrical center housing, as well as a front housing and a rear housing for closing the openings of the center housing. A known rotary electric machine having this housing structure includes a center housing, and front and rear housings. The center housing internally has a stator that is fastened by shrink or press fitting. The stator includes a stator core made from a laminated electromagnetic steel sheet, a stator coil wound around at least one slot on the stator core, and a rotor that is held rotatably and provided with a predetermined gap to the inner face of the stator. The rotor includes a rotor core made from a laminated electromagnetic steel sheet, a stator conductor fastened by shrink or press fitting or bonding to at least one slot on the rotor core, and a rotor shaft fastened by shrink or press fitting inside the inner face of the rotor. The rotor shaft is rotatably held via bearings provided on the front and rear housings, respectively. A protrusion of the shaft held at the front housing transmits motive power. Fastening the entire outer perimeter of the stator to the inner perimeter of the center housing through contact between them enables tight fastening for preventing the stator from being slipped by torque reaction or inertial force owing to vibrations acting on the stator. Placing the entire outer perimeter of the stator into contact with the inner perimeter of the center housing maximizes the contact area through which heat generated at the stator conductor is transferred to the center housing.
In recent years, there has been high demand for technologies with which to increase the output of rotary electric machines especially for use in EVs. Rotary electric machines with higher output are effectively achieved by increasing the thickness of the laminated stator or the amount of electric current. However, increasing the thickness of the laminated stator will make the rotary electric machine structure larger. Generally, a center housing and a stator have been fastened to each other by shrink or press fitting. A larger motor structure resulting from a thicker laminated stator increases shrink fitting time. This disadvantage has generated concerns about reduced production efficiency and upsized shrink fitting equipment. A thicker laminated stator also causes an increase in press fitting distance. This increases required press fitting force and necessitates upsizing of press fitting equipment. An increase in electric current flowing in a rotary electric machine increases the amount of heat generated at the stator conductor. Thus, forced cooling by a coolant or oil is necessary for cooling the rotary electric machine.
To resolve these problems, a structure for press fitting of a stator into a two-way split center housing is proposed, as disclosed in PTL 1. The center housing is split in two in the lamination thickness direction. The split center housing segments are partially tapered for matching. A rotary electric machine as is disclosed in PTL 2 has a structure that includes a coolant duct inside a center housing so as to cool the rotary electric machine by the forced circulation of a coolant in response to an increase in the amount of heat generated at the stator conductor. Another structure for coolant-use forced cooling is proposed, as disclosed in PTL 3. This structure includes a concave coolant duct provided on the outer perimeter of a center housing and a cylindrical cover provided as an outer casing on the outer perimeter of the center housing.